The present invention relates to a flame retardant polyamide composition that contains a triazine compound as flame retardant. The flame retarding effect of triazine compounds is thought to originate from the decomposition of these compounds at heating into nitrogen containing gaseous compounds that act as flame extinguishers. Melamine and melamine cyanurate have found widespread application in flame retardant plastics compositions. There are some publications in which more complicated melamine compounds and melamine salts are disclosed as flame retardant. However except melamine phosphate none of these compounds has found technical application.
The triazine compounds melamine and melamine cyanurate have the practical advantages that already at relatively low concentration adequate flame retardancy can be obtained without the use of synergists, no discoloration of the composition occurs under normal conditions and no corrosion of the processing apparatus is observed. However melamine and to a lesser extend melamine cyanurate have the disadvantage that during processing melamine is deposited in the mould, which may cause obstruction of the ventilation ducts and makes regular stops of the production process necessary to clean the mould. Although melamine cyanurate does show this disadvantageous phenomenon of mould deposit to a lesser extend than melamine, its use is further limited due to the fact that at higher temperatures, for instance at temperatures higher than 270xc2x0 C., the polyamide melts containing melamine cyanurate are unstable and show a decrease in melt viscosity. At temperatures of 290xc2x0 C. and higher in many cases gas evolution is so high that processing of compositions with melamine cyanurate is practically impossible. Therefore in the new class of high temperature engineering polyamides, for instance polyamide 4,6, melting temperature 290xc2x0 C., and the copolyamides, for instance PA 6,6/6,T/6,I with even higher melting temperatures, melamine cyanurate cannot be used and one has to rely on the halogenated organic compounds, for instance brominated polystyrene, to flame retard these polyamides. Compositions with these flame retardants usually are discolored and show low values for the comparative tracking index (CTI), which is a characteristic for the usefullness of polyamide compositions in applications such as for instance connectors and switches.
Object of the invention is therefore a flame retardant polyamide composition with a triazine compound as flame retardant, that possesses the favorable properties of melamine and melamine cyanurate but does not show the above mentioned disadvantages.
The inventors have found that the above requirements are completely fulfilled if the triazine compound as flame retardant is melam.
Melam is the condensation product of 2 molecules melamine. In German Patent DE-C-1694254 (priority 16.3.1968) the potential use of melam as flame retardant in polyamides is already mentioned as one of the compounds of the groups of melamine, melam and melamine-cyanurate and melamine derivates. Also in the publications NL-A-6915620, EP-A-0055893 en DE-A-3023965 melam is mentioned as possible flame retardant component in polyamide compositions. However all examples in these references have been limited to melamine and/or melamine cyanurate, and no single indication can be found of the extraordinary properties of melam for the solution of the technical problems mentioned above that are inherent to the use of melamine and melamine cyanurate as flame retardant in polyamide compositions.
Melam can be obtained by condensation of melamine in the presence of a catalyst. A known process is that described by V. A. Gal""perin et.al. in Zhurnal Organesheskoi Khimii, 7 (11), 2431-32 (1971) in which zincchloride is used as a catalyst. However as it is nearly impossible to remove the zincchloride sufficiently from the melam, the melam obtained by this process is hardly useful as flame retardant in processing caused by the presence of the residual catalyst in the melam. Therefore preference is given to a process in which melam is obtained, having a residual catalyst content that is not deliterious for the polyamide under processing conditions. Generally the residual catalyst content should be less than 2 wt %, preferably the catalyst content is less than 1 wt %, more preferable less than 0.5 wt % of the melam, best results are obtained if less than 0.2 wt % catalyst residue is present in the melam. Very suitable catalysts are organic acids or the melamine and/or ammonium salts thereof. Preferably the organic acid is a sulphonic acid for instance paratoluenesulphonic acid, that is present in a mole ration or preferably 0.1-3.0 to melamine. The reaction mixture is heated several hours while being stirred and under a nitrogen atmosphere. The temperature is chosen above 220xc2x0C. preferably between 280 and 320xc2x0 C. After cooling the melam salt of the sulphonic acid is obtained, and reacted with base into melam and freed from catalyst and base rests through repeated washing with water. As a base for instance ammonia, sodium hydroxide and/or sodium bicarbonate can be used.
In the condensation process generally also minor amounts of higher condensation products for instance melem and melon can be formed. The presence of these products however does not influence significantly the effect of the melam. Therefore if in this application the wording melam is used, then also melam containing minor amounts of higher condenstes of melamine is comprised. Minor has herein the meaning of in such quantity present that the special properties of melam as flame retardant for polyamide are not suppressed.
The polyamide of the polyamide composition is in principle not limited to a small group of polyamides. Both polyamides based on dicarboxylic acids and diamines and those derived from amino acids or lactams can be used in the compositions.
Examples of dicarboxylic acids are aliphatic dicarboxylic acids for instance oxalic acid, succinic acid,adipic acid and sebacinic acid, aromatic dicarbyoxylic acids, for instance tere- and isophthalic acid, naphthalene dicarboxylic acid and diphenylene dicarboxylic acid.
Examples of diamines are aliphatic diamines, for instance 1,4-diaminobutane, 1,6-diaminohexane, 1,5-methylpentanediamine and 2,2,4-trimethylhexanediamine, cycloaliphatic diamines, for instance diaminodicyclohexylmethane, and aromatic diamines, for instance p-phenylenediamine.
Useful polyamides are for instance, polypyrollidone (PA-4), polycaprolactam (PA-6), polycapryllactam (PA-8), polytetramethylene adipamide, (PA-4,6), poly(hexamethylene adipamide), (PA-6,6), poly(hexamethylene isophthalamide), (PA-6, I), poly(hexamethylene terephtalamide), (PA-6T), poly(metaxylylene adipamide), (PA-MXD,6), etc. and copolymers thereof for instance 6,6/6,T, 6,6/6,I, 6, 6/6,I/6,T etc.
Especially advantageous are polyamides having a melting temperture higher than 240xc2x0 C., preferably higher than 260xc2x0 C. It is even for the first time that for polyamides having a melting temperature of higher than 270xc2x0 C. a triazine derived flame retardant is demonstrated under practical conditions. Therefore poly(hoxamethylene adipamide), poly (tetramethylene adipamide), the copolyamides having a high melting temperature, for instance the copolyamides based on an aliphatic diamine, an aliphatic dicarboxylic acid and at least one aromatic dicarboxylic acid, are especially suited for the composition according to the invention. The molecular weight of the polyamide may vary over a wide range, but is preferably chosen such that processing in the melt is possible and that articles obtained from the melt have adequate mechanical properties. Generally the weight averaged molecular weight will be chosen in the range of 8000-60.000.
Depending on the required level of flame retardancy and the requirements with respect to mechanical properties the melam content of the composition may vary over a wide range, for instance between 1 and 40% by weight of the total composition. Below 1% the effect on flame retardancy is in general too small, above 40% the mechanical properties in general will be negatively influenced. Preferably the melam content is between 2 and 35% by weight, with more preference between 3 and 30% by weight. The low melam contents preferably are used in combination with a second flame retardant compound, for instance a halogen containing compound, such as brominated polystyrene or polyepoxyde. Other second compounds may be compounds having a suppressing effect on dripping of the composition in case of fire, for instance a low melting glass, examples thereof are leadborate and alkaliphosphate glass having a melting range of 200-14 600xc2x0 C. , preferably 300-500xc2x0 C., a zincborate preferably anhydrous zincborate, or an organic or inorganic fibrous material, for example fibrillar polytetrafluoroethylene and wollastonite. Special advantages are found in combinations with metaloxides, for instance antimony oxide and zinc oxide, which are especially useful in glass fibre reinforced compositions. However levels of these compounds should be kept low if a high arc tracking resistance is required.
The composition of the invention may further contain the usual additives, for instance stabilizers, colorants, processing aids, for instance mould release agents and flow improvers, fillers and the already mentioned reinforcing fibrous materials, heat and/or electricity conducting materials and other polymers, for instance (modified) elastomeric polymers for impact resistance improvement. Also chain extending agents for instance diepoxides may be present.
The composition according to the invention exhibits also excellent electrical properties. So is the arc tracking index, CTI, on a very high level and does no appreciable electric corrosion due to the flame retardant occur. Therefore electrical and electronic parts, obtained with the composition according to the invention, will fulfill the highest requirements. Especially electrical and electronic parts obtained from the polyamide compositions according the invention and having a melting temperature of at least 265xc2x0 C., preferably at least 275xc2x0 C., are exceptionally suited to be used in the surface mounting technique, SMT, in which they have to withstand very high temperatures during for instance infrared or hot air soldering. Examples of such parts are SIMM connectors (Single in line memory module).
The compositions can be obtained from the components by known methods as such. For instance the components may be dryblended and consequently fed into a meltmixing apparatus, for instance a Brabender kneader or an extruder. Also the components can be directly fed into a melt mixing apparatus and dosed together or separately. Preference is given to an extruder as meltmixing device. In that case the composition is obtained in pellets that can be used for further processing, for instance in injection moulding, extrusion or blow molding. Preferably the meltmixing is performed in an inert gas atmosphere and the materials are dried before mixing.
For a high effectivity of the melam as flame retardant it is advantageous to disperse the melam very finely into the polyamide matrix and to use herefor a double screw extruder. The particle size of the melam is chosen preferably as low as possible, for instance the particle size is  less than 100 xcexcm, preferably  less than 50 xcexcm with more preferance  less than 20 xcexcm. Most preferred is a particle size  less than 10 xcexcm. Especially if the polyamide composition is used in filaments, films and coating this small particle size is preferred.
The invention is further illustrated by the following experiments, without however being limited thereto.